So, the idea is that Vernon used to be an up and coming businessman, before he worked in Grunnings. His company sent him to France, to finalise a deal or something. This was before he hated anything that wasn't British. And who does he meet in France? an ethereally beautiful silvery blonde, who's currently going by the placeholder name of [Appoline's Sister]. They talk, and talk, and over the course of the two months or so that Vernon spends in France, they fall in love. [Appoline's Sister] at first didn't tell Vernon about magic, but a week or two before He's due to leave, she starts appearing at their meetups/date more and more exhausted and scared. Concerned, he asks her to open up to him. She tells him about magic, and how some of the purists in her social circles disapprove of her relationship with him, a muggle. (Vernon doesnt hate magic, he's actually really enamoured with the wonder of it.) [Appoline's Sister] tells him that she's still going to keep seeing him regardless, because she loves him, but she's scared that they might do something to him. He tells her he trusts her to protect him.
The day he applies for an extension of his work in France, so he can stay with her, she's found dead.
He meets her family for the first time then, when Appoline decides that he at least deserves to know what happened to [Appoline's Sister]. He's inconsolable, and when he asks how she died, she tells him she was killed by the same purists she'd been so scared of. Appoline's mother, kind of a purist herself, makes matters worse through some snide comments about how its Vernon's fault [Appoline's Sister] died, and how she should have known better to get with a muggle.
Vernon returns to Britain, heartbroken, with a newfound hatred of magic and those who use it.